first off i bought a built motor that was in a truck that was rolled over..for the next year the guy drove it around his property for misc things.like garbage,firewood.was his work horse.it was built by a local company called britco.when i drove the truck it ran fine.so i got the motor swap done.filled the rad went for a test drive.drove fine.the 2nd time i drove it,the temp hit 240-250 ish.let it cool down checked rad was half full.took 1.5gall of water to fill it..then i was goin to get exhaust done on it,it overheated again.so i bought a t-stat changed it.towed it home.pressure tested the rad,found out my lower rad hose was alil loose and my t-stat housing didnt seat right..fixed those 2.now its a 400 sbc stroked to a 406.has head work,estimated 450hp at the crank.so its a motor id really like to keep.it has headers on it.now just ideling itll hit 190-195,t-stat is a 195 hasnt gona above 200.just test drove it today it hit 230+.on my way back home i shut it off to coast about 2 blocks.started it back up was still up in the high temp.when i was pulling into the back yard to park it was at 190?im starting to thing that the headers are giving the temp sending unit false readings or its a head gasket..any thought would be great,thanks alot gals/guys take care

 

400 's always run alittle on the hot side. So its fine when idleing, but spikes up when driving? What kind of fan? Do you have the fan shroud on it?

 

same fan,rad,hoses that were on the 350 that stayed cool.same fan shroud,it is on it..it shouldnt be hittin 230-250,that just seems to hot to me..

 

thats way hot. perhaps the clutch went on the fan? that is unless youre running a flex fan. how many rows is your radiator? i would at least run a 3 row brass copper or a 2 row aluminum. as mentioned above the 400 runs hot. mine never had an issue around town or on the highway, but if i was in bumper to bumper it was all over. i was running a 3 row but i only had the upper shroud on. my clutch fan at the time seemed far from the radiator so i spaced it out still had an issue with bumper to bumper. before i got rid of the car i was going to get an electric fan to run on a toggle for those traffic issues. your car shouldnt be overheating if the t-stat, water pump, hoses, fan, and shroud are good. what heads are on this car? the 400 heads are supposed to be drilled for steam holes. what are your cam specs, intake, and carb? heres a link on clutch fans to help you check it out.
http://www.aa1car.com/library/cooling_fan_clutch.htm

 

it has a cluth fan on it,i dont suspect the fan because at idle for 20-30 min it stays cool,right at 190-195.i let it warm up to the normal temp then held it at about 2k for 5min.didnt over heat at all stayed right at normal temp.test drove it over heated.i got new head gaskets,valve cover,dizzy,exhaust.the works to do the head gasket.oil isnt creamy or anything,so i dont suspect the coolant is gettin into the crankcase.is it possible the head gasket gave just enough for the compression stroke to push air past the head gasket into the water flow?im still gettin air and bubbles out of the rad

 

how many row is your rad? my car had a one row in it and in stock 400 form it never ran hot. once i did h/c/i, and redid the bottom end it would run hot even with a new 3 row but only in traffic. it seems odd that it over heats only under a load. you have checked the radiator cap right? where is it dumping water from when it over heats. the overflow?